1. Field of the Invention
The present invention relates to a photothermographic material (hereinafter referred to as simply “photosensitive material”) and more specifically relates to a preferable photothermographic material for medical diagnosis, industrial diagnosis, industrial photography, printing and COM use.
2. Description of the Related Art
Recently, in the field of medical diagnosis films and photoengraving films, a strong need for reducing the volume of waste process liquid has arisen from the viewpoint of environmental preservation and space saving. Thus, a technology related to a photothermographic material as medical diagnosis film and photoengraving film has been desired, the material being such that it allows efficient light exposure with a laser image setter or laser imager, and providing a black image with a high resolution and sharpness. Such photothermographic material can provide the user with a more simple and environment-conscious thermal developing system using no solution-based process chemicals.
While a similar need has arisen in the field of general image forming materials, images used in the medical diagnosis field particularly require a high image quality such as excellent sharpness and graininess for fine depiction, and favor a blue-black tone for facilitating diagnosis. Although various hard copy systems using pigment or dye, exemplified as an inkjet printer and electronic photograph system, prevail as a general image forming system, none of these are satisfactory as an output system for medical images.
Organic silver salt-utilising thermally processed image forming systems are described in U.S. Pat. Nos. 3,152,904 and 3,457,075 as well as on page 279, Chapter 9, “Thermally Processed Silver Systems,” (Imaging Processes and Materials) Neblette, 8th edition, authored by D. Klosterboer, compiled by J. Sturge, V. Walworth and A. Shepp (1989).
In general, photothermographic materials are provided with a photosensitive layer in which a photo catalyst in a catalytically active quantity (for example silver halide), a reducing agent, a reducible silver salt (for example, organic silver salt) and an color tone modifier for controlling silver tone when necessary are dispersed in the matrix of a binder.
Photothermographic materials are heated to higher temperatures (for example, 80° C. or greater) after images are exposed to light to cause an oxidation-reduction reaction between or reducible silver salt (acting as an oxidizing agent) and reducing agent, thus providing black silver image. Silver halide produced on exposure to light catalytically acts on a latent image to promote the oxidation-reduction reaction, thus producing the black silver image on exposed areas. This process has been disclosed in various literatures including U.S. Pat. No. 2,910,377 and JP-B No. 43-4924.
Photothermographic materials have enjoyed a favorable response from the market due to the above-explained favorable characteristics, finding various applications, which also entails further improvement in performance. Among other things, it has been a great challenge to improve the coating quality and raise the productivity during the coating process.
In the case of conventional photosensitive materials which were developed by chemical solutions, since the film is swollen at the time of treatment with chemical solutions and dried again, a slight uneven thickness of the membrane due to variation in coating is eased and not recognized at the time of image formation. In contrast, in the case of photothermographic materials which do not undergo swelling due to treatment with chemical solution, a slight uneven thickness of the film at the manufacturing stage may result in an interference band depending on the species of light source to affect diagnosis imaging.
Cissing defects will affect diagnosis imaging and must be removed at the manufacturing stage. Thus, prevention of cissing defects is always an important challenge in improving the productivity. Cissing defects should be removed not only in preparing the image-forming side but also in preparing the back side.
Conventionally, a binder similar to the binder used for the surface having an image-forming layer has been used as a binder for the non-photosensitive back side layer of photothermographic materials. For example, when an organic solvent is used as a coating solvent, cellulose esters are used as a binder for protecting the image-forming layer and also as a binder for the back layer.
It has been proposed recently that water-soluble polymers such as gelatin and polyvinyl alcohol are used on either side of the image-forming layer for the purpose of increasing film strength and image storability. Use of gelatin is desired because when gelatin is used as a binder, high-speed coating properties can be particularly improved, in addition to an increase in said properties. It has been known that polymer latexes are added for easing rigidity and imparting flexibility as a film, when gelatin is used as a binder. Since polymer latexes with a less glass transition temperature are more effective in imparting flexibility, commonly used are latexes such as polyethylacrylate with glass transition a temperature of −20° C. or less.
However, in photothermographic materials that are thermally developed at high temperatures (approximately 80 to 250° C.), an edge of the photosensitive material is curled upon thermal development, thus potentially making the development uneven and affecting development uniformity.
Further, photothermographic materials often include a dye that can be discolored upon thermal development so as to avoid color remaining and to attain clearness of an image. Thus, especially when such a discolorable dye is included in a photothermographic material, the discoloring effect does not work well on the edge, raising a problem of residual color.
Under these circumstances, the technology for controlling the curl of photosensitive materials upon thermal development is important and has been long awaited.
JP-A No. 11-352625 has described a non-photosensitive layer containing a dye that can be bleached with a base precursor and technology by which a water-soluble polymer such as polyacrylamides and dextrans as preferable examples are contained in a composition layer on the same side of the non-photosensitive layer. However, the present inventor evaluated the technology, finding that it was not very effective in improving residual color and unable to solve the above problems.
Conventionally, dyes are added to photography photosensitive materials for controlling filtration property and preventing halation or irradiation. In particular, medical diagnosis imaging requires very minute depiction, a high-quality image has been long desired that is excellent in sharpness and graininess. Thus, it is a common practice that these dyes provide functions on image exposure and are completely removed during development processes so that the image is not colored by absorption of the dye at a visible region after the image is formed.
In conventional wet developing processes, it is possible to elute out the dyes from a photosensitive material into a processing liquid, thus making it relatively easy to remove the dye from the photosensitive material. However, said removal is difficult in dry developing processes such as thermal development. Therefore, such a method has been already proposed that discolors a dye by using heat upon thermal development. For example, JP-A No. 11-352626 has disclosed a technology in that the melting point of base precursor is controlled to generate a base upon thermal development, thus attaining discoloring of the dye.
However, this technology is insufficient in discoloring the dye when photosensitive materials are much speedily subjected to thermal development, and is disadvantageous in that the color remaining of a dye occurs in the photothermographic material after processing.
Further, adding an excessive base precursors for improving the discoloration could cause another problem: heat during storage gradually deteriorates dye's absorption to improve filtration property and to prevent halation and irradiation when photosensitive materials are stored in an environment at high temperature.